/Engine/Source/Runtime/GeometryCore/Public/DynamicMesh/DynamicMeshOverlay.h
Create a new copy of ElementID, and update connected triangles in the TrianglesToUpdate array to reference the copy of ElementID where they used to reference ElementID (Note: This just calls "SplitElementWithNewParent" with the existing element's parent id.)
the ID of the newly created element
int SplitElement ( int ElementID, const [TArrayView](API\Runtime\Core\Containers\TArrayView)< const int > & TrianglesToUpdate )